The invention described herein was made in the performance of work under a NASA contract and is subject to the provisions of Section 305 of the National Aeronautics and Space Act of 1958, Public Law 85-568 (72 Stat. 435; 42 U.S.C. 2457).
1. Field of the Invention
The present invention relates generally to detecting metal subterranean anomalies such as land mines and, more particularly, to the use of a digitally configured system for processing complex system transfer data utilizing one or more transmitter and receiver sets.
2. Description of Prior Art
On average, every twenty minutes someone in the world loses a limb to a landmine. Landmines may be made of plastic or metal. In just the Sinai desert and the West Bank of the Nile River, approximately 10,000,000 metal mines remain from World War II. Shifting sands and the like result in mines being widely displaced from their original locations. Thus, it is desirable that a mine detector be highly sensitive to permit location of small metal objects such as miniature mines at various depths. In fact, it would be desirable to provide a metal detector that is able to locate underground mines four or more times deeper than is believed now possible utilizing the most widely used prior art metal detector.
Moreover, it would be desirable to provide a landmine detector that may be utilized for detecting either plastic or metal land mines. U.S. application Ser. No. 09/056,363, filed Apr. 7, 1998, discloses an exemplary plastic mine detector. The present application discloses a very high sensitivity metal detector that may be utilized in conjunction with the exemplary plastic mine detector mentioned above or with other plastic detectors with little or no interference problems. Such interference problems may be produced by additional electronic components, antennas, metal connectors, or other metal objects, in the immediate vicinity of the metal detector, such as between transmitter/receiver coils, that otherwise result in transmitter and/or receiver noise and variations that create false signals.
Patent applications that show attempts to solve problems related to the above include the following:
U.S. Pat. No. 5,786,696, issued Jul. 28, 1998, to Weaver et al., discloses a metal detector which utilizes digital signal processing and a microprocessor to process buffers of information which is received at a periodic rate. Both high and low gain phase quadrature and in-phase signals are provided via a multiplexer to an analog-to-digital converter from a first receive antenna. A second receive antenna provides phase quadrature and in-phase signals also through the multiplexer to the analog-to-digital converter. The received signals are averaged, decimated and low pass filtered to eliminate noise and reduce the quantity of data which must be processed. A threshold (triggering) processing operation is performed to determine whether a valid target signal is present in the data. If not, no further processing is performed. The in-phase and quadrature components are processed using Fourier transforms to select a frequency band which includes the energy for the target signal. The energy in this frequency band is utilized to determine the identification of the target. The depth of the target is determined by comparing the quadrature phase components received from the first and second receive antennas. The size of the target is determined by reference to a look-up table based on the depth factor and the signal amplitude determined for the target object A display screen has a plurality of horizontal depth symbols, each of which has a plurality of size indicators and upon determining the depth and size of a target, one depth symbol is activated together with one of the size indicators to concurrently display this information to an operator.
U.S. Pat. No. 5,729,143, issued Mar. 17, 1998, to Tavernetti et al., discloses a metal detector which includes a receive coil and a transmit coil connected in an inductive bridge. To overcome imbalances in the bridge due for instance to misalignment of the coils or the presence of mineralization in the medium which is being examined, the metal detector automatically produces a nulling (bucking) signal to cancel out the effects of any unwanted receive coil signals detected during calibration. This nulling signal is a nulling current both in terms of level and phase, and its level and phase are determined during a calibration process prior to actual metal detection. By inclusion in the metal detector of a microprocessor (microcontroller) operating at a much higher frequency than the variations in the magnetic field used to detect metal, the nulling signal generation is performed with a high degree of time resolution, resulting a precision metal detector which adaptively ignores any unwanted signals.
U.S. Pat. No. 6,163,292, issued Dec. 19, 2000, to Liedtke et al., discloses a method of determining the ability of a medium to absorb electromagnetic waves including placing an antenna unit having spaced transmitting and receiving antennas on a limiting surface of a medium, emitting, with the transmitting antenna, a radar wave into the medium which is detected as a cross-signal by the receiving antenna, pre-processing and digitizing the cross-signal, and, thereafter, analyzing the cross-signal with an algorithm for determining the ability of the medium to absorb electromagnetic waves and, thereby, a type of the medium; and an electromagnetic sensor the operation of which is based on the method.
U.S. Pat. No. 6,150,810, issued Nov. 21, 2000, to Lyle G. Roybal, discloses a method for detecting a presence or an absence of a ferromagnetic object within a sensing area which may comprise the steps of sensing, during a sample time, a magnetic field adjacent the sensing area; producing surveillance data representative of the sensed magnetic field; determining an absolute value difference between a maximum datum and a minimum datum comprising the surveillance data; and determining whether the absolute value difference has a positive or negative sign. The absolute value difference and the corresponding positive or negative sign thereof forms a representative surveillance datum that is indicative of the presence or absence in the sensing area of the ferromagnetic material.
U.S. Pat. No. 5,969,528, issued Nov. 19, 1999, to Brent C. Weaver, discloses a metal detector which has multiple transmit and receive coils for producing multiple detection fields. In one embodiment, a transmit coil is combined with two receive coils in a configuration that enables the detector to generate two detection fields, one being substantially narrower than the other. The transmit coil is inductively balanced with the receive coils such that the transmit coil induces minimum signals in each of the two receive coils. A metal target lying within a detection field changes the coupling between transit and receive coils and produces signals in the receive coils. The received signals are utilized to identify the target""s presence within one or both of the detection fields. The use of two detection fields, substantially different in size, enables the metal detector to search over a broad area for object detection and then narrow the search to more precisely locate the detected object. A further embodiment has two transit coils and one receive coil and likewise produces a broad and a narrow detection field.
U.S. Pat. No. 5,790,685, issued Aug. 4, 1998, to Bradley T. Sallee, discloses an invention which relates to the field of metal detectors. More particularly, it relates to an imaging metal detector for imaging the metal on subjects passing through a spatial plane providing the specific location, shape and mass of the metal object. This invention makes use of an array of active sensors to transmit and receive magnetic beams and a computer for generating an image of the metal object based upon the data received from the sensors. Through the use of this invention it is possible to scan several subjects at the same time and generate a two- or three-dimensional image of any metal object on the subject as well as precise location, mass and type of metal contained in the object.
U.S. Pat. No. 5,715,320, issued Feb. 3, 1998, to Allie et al., discloses an active adaptive control system which introduces a control signal from an output transducer to combine with the system input signal and yield a system output signal. An error transducer senses the system output signal and provides an error signal. An adaptive filter model has a model input from a reference signal correlated to the system input signal, and an output outputting a correction signal to the output transducer to introduce the control signal. Performance of the model is selectively controlled to control the signal sent to the output transducer. Various monitoring and control methods are provided, including spectral leak signal monitoring and control, correction signal monitoring and control, frequency responsive spectral transfer function processing of the leak signal and/or the correction signal, reference signal processing, and fuzzy logic control.
U.S. Pat. No. 5,694,133, issued Dec. 2, 1997, to Rabindra N. Ghose, discloses an invention which relates to automatic direction finding methods and systems for an electromagnetic or acoustic signal source by utilizing the concept of adaptive nulling of a narrow or wideband signal received by one sensor from the same signal received by the other, along with appropriate adjustments, in a two-sensors-interferometer. Plurality of sensors can be used to measure two orthogonal angles of arrival of a signal. Inventive methods and devices permit singling out a particular signal of interest among others by allowing a frequency tuning and selective amplification of the particular signal of interest without destroying the differential phase or time delay between the signals received by the sensors that determine the angle measurement. A high degree of signal cancellation during the adaptive nulling process permits a correspondingly high degree of accuracy and resolution for the angle measurement and also permits a relatively short baseline for the interferometer.
U.S. Pat. No. 5,565,771, issued Oct. 15, 1996, to Hamelin et al., discloses a magnetic testing device for detecting loss of metallic area and internal and external defects in elongated magnetically-permeable objects and includes a permanent magnet assembly having poles adapted to be spaced apart in the longitudinal direction of an elongated object for inducing a longitudinal magnetic flux in a section of the object between the poles of the magnet assembly, the magnet assembly being strong enough to magnetically saturate the section of the object. A tubular pole piece is arranged to surround the object adjacent each pole of the permanent magnetic assembly for directing the magnetic flux into the object at one pole and out of the object at the other pole. Hall effect devices are placed in the path of the magnetic flux for sensing the reduction of the flux passing through the elongated object due to any reduction of cross-sectional area of the elongated object between the pole pieces caused by loss of metallic area in the elongated object. A leakage flux sensor is installed between the pole pieces for detecting an external and internal defects in the objects. Circuitry is provided for increasing the linear resolution of the metallic area measurement of the object, whereby signals obtained correspond more closely to the variations in the metallic area of the elongated object. Preferably, the circuitry combines the local fault (LF) signal with the loss of metallic area (LMA) signal to benefit from both the high linear resolution of the LF signal and the long range stability of the LMA signal to improve the resolution of the LMA signal.
U.S. Pat. No. 5,537,041, issued Jul. 16, 1996, to Bruce H. Candy, discloses an invention which utilizes a method of an apparatus to perform metal discriminatory detection comprising the application of a multi period rectangular wave form to a transmitter coil, processing received signals by combining received components measured during different selected periods with respect to the transmitted signal in a manner based upon the predictable characteristics of ground constituents and thereby to provide an output signal which will be selectively indicative of a metal object within a target volume and substantially distinguishable from any signal arising from any electrically non-conducting ferrite in the target volume.
U.S. Pat. No. 5,065,098, issued Nov. 12, 1991, to Salsman et al., discloses a locating system which has its performance enhanced by the use of digital filtering. The locating system may include a transmitter for transmitting an electromagnetic signal from a concealed object. The locating system may also use electromagnetic signals from existing electromagnetic sources unrelated to the locator system. A receiver is provided for receiving a selected electromagnetic signal. The signal is subsequently converted from an analog signal into a digital signal and subjected to digital filtering. The resulting digital signal is then processed to provide location information about the concealed object.
U.S. Pat. No. 4,709,213, issued Nov. 24, 1987, to Robert J. Podhrasky, discloses a metal detector circuit which includes a transmit coil and a receive coil arranged in a balanced induction configuration in an electromagnetic field. The receive signal from the receive coil is input to electronic switches which receive quadrature reference inputs from a phase shift circuit. The phase demodulated outputs of the switches are passed through amplifiers and input to an analog-to-digital converter to produce digital signal samples which are transmitted through a bus. The bus is connected to random access memory and a read only memory which includes a stored signal processing program. A microprocessor is connected to the bus for receiving the digital signal samples and the stored program from memory. The microprocessor executes the stored signal processing program to produce a digital output signal which is transmitted through the bus to a digital-to-analog converter. The converter produces an analog output signal which is passed to an output driver circuit which produces a target indication signal at a speaker. The digital signal processing provided by the microprocessor includes concurrent ground cancellation and discrimination without the need for operator selection of these functions. The digital output produced by the microprocessor, upon detection of an object in the electromagnetic field, can be displayed in digital readouts.
U.S. Pat. No. 4,678,992, issued Jul. 7, 1987, to Allen W. Hametta, discloses a metal detector circuit comprising an oscillator capable of sensing any type of metal in motion over a predetermined frequency. The oscillator supply voltage is derived from a feedback correction circuit which compensates for stationary metal as well as metal moving in a lower frequency past the sensing coil of the oscillator by adjusting the supply voltage of the oscillator to make up for Q losses. The higher frequencies are blocked by a DC time constant (low pass filter) and the sensor coil is able to see through stationary metal and detect a moving metal target on the other side thereof. The amplitude of the modulated signal from the oscillator remains nearly constant throughout its detection range by this supply voltage feedback correction circuit. The detection circuit also provides a reliable output signal indicating when the sensor portion is damaged due to an open or shorted coil circuit.
The above applications do not show a highly sensitive, noise resistant, digitally-based magnetic transceiver with a unique configuration of transmitter/receiver coils for operating with other electronic equipment, such as a plastic mine detector, and utilizing a digitally derived spectral transfer function based on in-phase and quadrature signals. Those skilled in the art have long sought and will appreciate the present invention that addresses these and other problems.
Accordingly, the present invention provides a method operable for detecting materials, such as metal, within an environment utilizing a transceiver which may comprise one or more steps such as, for instance, digitally producing a transmit signal, transmitting the transmit signal through the environment, receiving a first received signal through the environment, digitizing the first received signal, determining reference digital data related to a reference transfer function of the transceiver for a magnitude and phase of the transmit signal and the first received signal, receiving a subsequent received signal through the environment, determining subsequent digital data related to a subsequent transfer signal of the transceiver for a magnitude and phase of the transmit signal and the subsequently received signal, and comparing the reference digital data to the subsequent digital data to detect the material.
The step of forming the reference transfer function may further comprise producing an estimated reference transfer function at each of a plurality of iterations, stacking a plurality of estimated reference transfer functions, and/or utilizing an in-phase and a quadrature replica of the magnitude and the phase of the transmit signal to determine the reference transfer function. Other steps may include iteratively determining a nulling signal which minimizes the power of the sum of the first received signal and the nulling signal and/or determining the reference transfer function utilizing a magnitude of an in-phase nulling signal and a magnitude of a quadrature nulling signal over a selected frequency range in combination with a phase of the in-phase nulling signal and a phase of the quadrature nulling signal.
The method may comprise determining the reference transfer function and the subsequent transfer function over a selected frequency spectrum, utilizing at least one transmit coil for the transmitting, utilizing at least one receive coil for the receiving, switching between at least two receive coils such that each receive coil has an associated reference transfer function and/or storing the reference transfer function in memory.
In another embodiment, the method may comprise one or more steps such as digitally producing a transmit signal, utilizing at least one transmit coil for transmitting the transmit signal through the environment, utilizing at least one receive coil for receiving a first received signal through the environment, digitizing the received signal, digitally forming a reference signal related to a magnitude and phase of the transmit signal and the first received signal over a frequency spectrum, storing the reference signal in a memory, receiving a second received signal through the environment, digitally forming a second signal related to a magnitude and phase of the transmit signal and the second received signal over the frequency spectrum, and comparing the reference signal to the second signal to detect the material wherein the reference signal is related to the reference spectral transfer function of the transceiver.
The method may further comprise initializing the transceiver by digitally forming the reference signal. The second signal is then related to a second transfer function of the transceiver determined after the initialization of the transceiver. Other steps may include digitally producing a periodic signal such as a sinusoidal sweep signal and may include amplitude modulating the sinusoidal sweep signal.
In yet another embodiment, the method may comprise one or more steps such as providing a plurality of transmit coils for transmitting at least one transmit signal through the environment, providing a plurality of receive coils for receiving at least one received signal through the environment, determining at least one digital reference signal related to at least one spectral transfer function corresponding to the at least one transmit signal and the at least one received signal, storing the at least one digital reference signal in memory, receiving at least one subsequently received signal through the environment, determining at least one subsequent digital signal related to at least one subsequent spectral transfer function corresponding to the at least one transmit signal and the at least one subsequently received signal, and comparing the at least one digital reference signal to the at least one subsequent digital signal to detect the material.
Other steps may comprise providing a first set of transmitter/receiver coils, providing a second set of transmitter/receiver coil, determining a first digital reference signal related to the first set of transmitter/receiver coils, and determining a second digital reference signal related to the second set of transmitter/receiver coils. Additionally, the method may comprise providing that the first set of transmitter/receiver coils is related to detecting the material in a first region of the environment, and providing that the second set of transmitter/receiver coils is related to detecting the material in a second region of the environment. In one embodiment the method may comprise providing that the first set of transmitter/receiver coils are offset from the second set of transmitter/receiver coils but are positioned within a plane common to both first set of transmitter/receiver coils and the second set of transmitter/receiver coils. Additionally, the method may comprise switching between the first set of transmitter/receiver coils and the second set of transmitter receiver coils.
Thus the present invention provides for a magnetic transceiver system for detecting material within an environment which may comprise one or more elements such as, for instance, a digital waveform generator to produce a digital transmit waveform, a digital-to-analog converter to convert the digital transmit waveform to an analog transmit signal, at least one transmit sensor for coupling the analog transmit signal to the environment, a receive sensor for receiving the analog transmit from the environment to provide an analog received signal, an analog-to-digital converter for converting the analog received signal to a digital received signal, a module for comparing the digital received signal with the digital transmit waveform to produce digital data related to a transfer function of the transceiver within a reference portion of the environment over a selected frequency range, a reference generator for storing reference digital data related to a reference transfer function of the transceiver over the selected frequency range, an error generator for comparing the reference digital data to a subsequently produced digital data related to a subsequently measured transfer function of the transceiver within a portion of the environment to be measured for the material.
Other elements of the invention may comprise an electronics section mounted between the at least one transmit coil and the at least one receive coil. The module may comprise a digital signal processor. The transceiver may comprise a first set of transmitter and receiver coils, and a second set of transmitter and receiver coils wherein the first set of transmitter and receiver coils is separately operable from the second set of transmitter and receiver coils.
Other elements may comprise an audio module connected to the error generator and/or a current driver amplifier for driving the at least one transmit sensor.
An object of the present invention is to provide an improved metal detector that may be utilized in an environment with other electronic equipment.
This and other objects, features, and advantages of the present invention will become apparent from the drawings, the descriptions given herein, and the appended claims. It will be understood that any listed objects of the invention are intended only as an aid in understanding aspects of the invention and are not intended to limit the invention in any way.